SAW components use acoustic waves which travel at the speed of sound. The SAW components are preferred over widely used transmission line components because acoustic waves have a substantially shorter wave length at operating frequency than electromagnetic waves which travel at the speed of light. Therefore, for a given operating frequency, a SAW resonator filter provides a smaller sized structure than a transmission line structure, therefore, making them suitable for miniaturized radio frequency applications. Furthermore, SAW structures are easily integratable with other active circuits such as amplifiers and mixers which are produced using conventional integrated circuit technologies.
For the above reasons, the popularity of SAW structures in radio frequency applications has been steadily increasing, especially in resonator filter applications. FIG. 1 depicts the diagram of a conventional SAW resonator filter structure 100 which includes a SAW transducer 110, a pair of reflectors 120, and a piezoelectric substrate 105. The SAW filter structure 100 is connected to a voltage source Vs having a resistive load Rs. The SAW transducer 110 comprises a first electrode 112 having a first set of open-ended fingers 114 and a second electrode 116 having a second set of open-ended fingers 118. The first electrode 112 and the second electrode 116 comprise conductive layers patterned on the piezoelectric substrate such that a first set of fingers 114 and a second set of fingers 118 are interdigitated in relation to each other, as illustrated.
Conventional SAW resonator filters have an inherently narrow bandwidth when designed for resistive termination. The bandwidth is limited because of the low piezoelectric coupling coefficient of most substrates. However, in some radio frequency applications, such as those used for receiver selectivity, a substantially wide bandwidth is desired. In order to achieve the widest possible bandwidth, it is necessary to provide large numbers of fingers in the SAW transducer 110. It is well known that the widest bandwidth is achieved with a resistive source load Rs whose resistance is equal to the capacitive reactance at the operating frequency of the capacitors formed by the interdigitated fingers 114 and 118. As the number of interdigitated fingers increases for achieving the widest bandwidth, so does the interdigital capacitance between the electrodes 112 and 116, thereby reducing the optimum termination resistance. However, when the SAW structure terminates an active stage, it is very desirable to have a high termination impedance in order to realize high gains utilizing very little bias current drain. The current drain is an important parameter in miniaturized receivers where SAW structures are used. Therefore, it is desired to achieve the widest bandwidth in a SAW filter structure while presenting a sufficiently high termination impedance.